German Patent Laid-Open Publication No. 4,405,092C1 discloses a brake booster apparatus which has a solenoid coil for establishing automatic brake actuating operation. The brake booster includes a housing, a plurality of movable walls, a power piston, an input rod, an input member, a plunger member, a pressure control valve and a solenoid coil. The movable walls form front and rear constant pressure chambers, and front and rear variable pressure chambers. The variable pressure chambers are selectively communicated with the constant pressure chambers in response to the operation of the pressure control valve. The constant pressure chambers are communicated with an intake manifold (negative pressure source) of the engine at all times. The power piston is fixed to the movable walls, and in response to a pressure difference between the constant pressure chambers and the vacuum pressure chambers, the position of the movable walls changes. According to the movement of the movable walls, the power piston is actuated.
The pressure control valve is comprised of an atmospheric control valve and a negative pressure control valve. The atmospheric control valve controls communication between the variable pressure chambers and the atmosphere. The negative pressure control valve controls communication between the constant pressure chambers and the variable pressure chambers. Under the energized condition of the solenoid coil, the solenoid coil actuates the plunger member and the power piston, and the brake booster generates an operating force of the brake master cylinder.
The operation of the conventional brake booster can be seen with reference to FIG. 9 which graphically illustrates the relationship between the input force and the output force. The vertical axis represents the output value of the brake booster, and the horizontal axis represents the depression force of the brake pedal. The broken line C represents when the solenoid coil is energized. When the brake pedal is depressed with a force f.sub.1, the atmospheric control valve is operated to an open position. As a result, the movable walls and the power piston are operated and an output force corresponding to the point a.sub.1 is generated.
When the brake pedal has been rapidly depressed in an emergency situation, the solenoid coil is energized to attract the plunger member. The power piston is operated in response to the actuation of the solenoid coil. As a result, an output force corresponding to the point C.sub.1 is generated. After this time, the brake booster generates an output force along the broken line C. Under the condition where the depression force f.sub.1 is applied, when the solenoid is energized at point a.sub.1, the output force changes to point C.sub.1.
The above mentioned conventional brake booster suffers from a variety of disadvantages. In one respect, due to the ON-OFF controlling solenoid, the brake booster cannot generate a continuous variable output in response to the input force. More specifically, the brake booster is not able to generate an appropriate braking force in response to the load of the vehicle. Further, an appropriate output brake force cannot be generated to effect an automatic braking operation. Furthermore, when the brake pedal is released, the diminution of the braking force is not responsive to the driver's intention.